Without limiting the scope of the present disclosure, its background will be described with reference to evaluation of hydrocarbon bearing subterranean formations, as an example.
It is well known in the subterranean well drilling and completion art to perform tests on formations intersected by a wellbore. Such tests are typically performed in order to determine geological or other physical properties of the formation and fluids contained therein. For example, parameters such as permeability, pore pressure, porosity, fluid resistivity, directional uniformity, temperature, pressure, bubble point and fluid composition may be determined. These and other characteristics of the formation and fluid contained therein may be determined by performing tests on the formation before the well is completed.
One type of tool used for testing formations includes an elongated tubular body divided into several modules serving predetermined functions. For example, the testing tool may have a hydraulic power module that converts electrical into hydraulic power, a telemetry module that provides electrical and data communication between the modules and an uphole control unit, one or more probe modules that collect samples of the formation fluids, a flow control module that regulates the flow of formation and other fluids in and out of the tool and a sample collection module that may contain one or more chambers for storage of the collected fluid samples.
The probe modules may have one or more probe-type devices that create a hydraulic connection with the formation in order to measure pressure and take formation samples. Typically, these devices use a toroidal rubber cup-seal, which is pressed against the side of the wellbore while a probe is extended from the tester in order to extract wellbore fluid and affect a drawdown. The rubber seal of the probe is typically about 3-5 inches in diameter, while the probe itself is only about half an inch to an inch in diameter. It has been found, however, that due to the small area contacted by such probes, a hydrocarbon deposit or other valuable information may be missed.
Attempts have been made to overcome the above sampling limitations using, for example, straddle packers in association with a downhole formation testing tool. The straddle packers are inflatable devices typically mounted on the outer periphery of the tool and can be placed as far as several meters apart from each other. When expanded, the packers isolate a section of the wellbore and samples of the formation fluid from the isolated area can be drawn through one or more inlets located between the packers. Although the use of straddle packers may significantly improve the flow rate over the conventional probe-type devices described above, the straddle packer type testing tools also have several important limitations. For example, the volume of fluid between the straddle packers results in long clean up time and, even after clean up, the samples are not obtained directly from the formation.
Therefore, a need has arisen for an improved downhole formation testing and sampling apparatus that is operable to provide an accurate estimate of a reservoir's producibility. A need has also arisen for such an improved downhole formation testing and sampling apparatus that is operable to provide a large exposure volume without requiring a long clean up time. Further, a need has arisen for such an improved downhole formation testing and sampling apparatus that is operable to obtain fluid samples directly from the formation.